A hydraulic drive system that performs load sensing control so that delivery pressure of a hydraulic pump is higher by target differential pressure than maximum load pressure of a plurality of actuators is called a load sensing system. Patent document 1 discloses an exemplary load sensing system.
The hydraulic drive system disclosed in patent document 1 includes an engine, a variable displacement hydraulic pump, a plurality of actuators, a plurality of flow and directional control valves, a detecting circuit, a control means, and an unloading valve. Specifically, the variable displacement hydraulic pump is driven by the engine. The actuators are driven by hydraulic fluid delivered from the hydraulic pump. The flow and directional control valves control a flow rate of the hydraulic fluid supplied from the hydraulic pump to the actuators. The detecting circuit detects the maximum load pressure of the actuators. The control means performs the load sensing control so that the delivery pressure of the hydraulic pump is higher by target differential pressure than the maximum load pressure of the actuators. The unloading valve is disposed in a line that connects the hydraulic pump to the flow and directional control valves. The unloading valve opens when the delivery pressure of the hydraulic pump is higher than the sum of the maximum load pressure and the unloading set pressure, to thereby return the fluid delivered from the hydraulic pump, thereby limiting an increase in the delivery pressure of the hydraulic pump.
Patent document 2 discloses a load sensing system having an exhaust gas purifying device. The exhaust gas purifying device of this system is disposed in an exhaust pipe and includes an exhaust resistance sensor. When a value detected by the sensor is equal to or higher than a predetermined level, a control device outputs a signal to thereby control a regulator of the main pump and an unloading valve. A delivery rate and a delivery pressure of the hydraulic pump are thereby simultaneously increased to apply hydraulic load to an engine. The output of the engine is thus increased to thereby increase an exhaust gas temperature. An oxidation catalyst is thus activated to thereby burn filter deposits and regenerate the filter.